Letter
www.acsami.org
Self-Templated Synthesis of Mesoporous Carbon from Carbon
Tetrachloride Precursor for Supercapacitor Electrodes
Duihai Tang, Shi Hu, Fang Dai, Ran Yi, Mikhail L. Gordin, Shuru Chen, Jiangxuan Song,
and Donghai Wang*
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United
States
S Supporting Information
*
ABSTRACT: A high-surface-area mesoporous carbon material has been
synthesized using a self-templating approach via reduction of carbon
tetrachloride by sodium potassium alloy. The advantage is the reductiongenerated salt templates can be easily removed with just water. The produced
mesoporous carbon has a high surface area and a narrow pore size distribution.
When used as a supercapacitor electrode, this material exhibits a high specific
capacitance (259 F g−1) and excellent cycling performance (>92% capacitance
retention for 6000 cycles).
KEYWORDS: mesoporous carbon, self-templating, carbon tetrachloride, sodium potassium alloy, supercapacitor
M
As to the templates for the synthesis of the mesoporous
carbon, there are two major categories: supramolecular
aggregates such as surfactant micelle arrays (soft template)
and preformed mesoporous solids (hard template) such as
SiO2, CaCO3, and MgO.17−19 In the synthesis, the organic
precursors are directed into organized structure by the
templates, carbonized at high temperature, and then the
templates are completely removed by chemical (leaching,
etching) or physical (thermal treatment) methods to obtain the
mesoporous carbon.20 Additionally, some activation-free
strategies have also been developed to synthesize porous
carbon.21,22 However, the template-free methods to synthesize
mesoporous carbon are rarely reported and thus desirable.
In this report, we introduce a new synthesis procedure of
mesoporous carbon without any external templates. The
mesoporous structure is generated by a self-templating method
using reaction byproducts as templates and carbon tetrachloride
as precursors. The synthesis is via reduction of liquid carbon
tetrachloride by sodium potassium alloy (NaK, liquid) and selfassembly of the reduction-generated carbon atoms around the
as-generated salt byproduct.23 The advantage is the reductiongenerated salt templates can be easily removed with just water.
The produced mesoporous carbon has a high Brunauer−
Emmett−Teller (BET) surface area of 2012 m2 g−1 and a
narrow pore size distribution, with an average pore diameter of
esoporous carbon materials are of interest in many
applications thanks to their high surface area and
favorable physicochemical properties,1 including electrical
conductivity, thermal conductivity, chemical stability, and
mechanical stability.2 They have been widely applied in gas
storage, water purification, catalyst supports, and electrode
materials for fuel cells, batteries, and supercapacitors3 The
preparation of mesoporous carbon usually involves hightemperature pyrolysis of organic precursors with different
types of templates. A wide variety of precursors have been used
to prepare the mesoporous carbon.4 In 1999, Ryoo’s group
reported the preparation of ordered mesoporous carbon by
carbonization of sucrose inside mesoporous silica template. The
mesoporous carbon could be obtained after the removal of the
silica template by using sodium hydroxide aqueous solution.5
Since then, many other organic precursors have been utilized,
including petroleum pitches,6 resins (phenol-formaldehyde and
resorcinol-formaldehyde resins),7 alkene (ethylene and propylene),8,9 aromatic precursors (naphthalene, anthracene, pyrene,
benzene, styrene, and acenaphthene),10,11 and other small
organic molecules (furfuryl alcohol and acetonitrile).12 Carbon
tetrachloride (CCl4) has also been used as precursor for
preparation of nonporous carbon material. For example, Li and
Qian reported synthesis of diamond through the reaction
between CCl4 and sodium at 700 °C.13 Numerous research
works have subsequently been conducted focusing on the
reactions of alkali metals and halocarbon to produce carbon
materials.14−16
© 2016 American Chemical Society
Received: December 14, 2015
Accepted: February 25, 2016
Published: February 25, 2016
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DOI: 10.1021/acsami.5b12164
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Letter
ACS Applied Materials & Interfaces
4 nm. Benefiting from its high surface area and mesoporous
structure, the mesoporous carbon was evaluated as an electrode
material for supercapacitors and exhibits high specific
capacitance (259 F g−1 at 1 A g−1), and high rate stability
(189 F g−1 at a higher current density of 100 A g−1).
The preparation of this mesoporous carbon was carried out
in a glovebox filled with Ar (see details in Supporting
Information). The synthetic route is schematically shown in
Figure 1 NaK alloy was slowly added to a toluene solution of
Figure 1. Synthetic route of the mesoporous carbon.
Figure 2. (a) XRD pattern of MC and (b) Raman spectrum of MC.
carbon tetrachloride (CCl4) by syringe. This mixed solution
was vigorously stirred and heated at 120 °C for 8 h, with the
production of black particles. Then hydrogen chloride solution
(2 M in diethyl ether) was added slowly into the mixture. The
intermediate products were collected and carbonized at 700 °C
(heated up at the rate of 15 °C min−1) for 40 min under Ar
flow. These products were washed with excess DI water, and
collected by filtration. Finally, the as-prepared mesoporous
materials were dried at 80 °C under vacuum for 12 h before
use. These mesoporous materials were named as MC.
X-ray powder diffraction (XRD) pattern of MC is shown in
Figure 2a. The broad and low intensity diffraction peaks at
26.3° and 43.8° indicate the low degree of graphitization and
amorphous nature of the final product.24 The Raman spectrum
of the final product (Figure 2b) has two broad peaks at around
1330 and 1600 cm−1, which correspond to the D-band and the
G-band, respectively. The higher intensity of the D band
compared to the G band further proves the disordered atomic
structure of MC.25
Nitrogen adsorption/desorption isotherm (Figure 3a)
confirms the mesoporous structure of MC. The BET specific
surface area of MC reaches 2012 m2 g−1, and the pore volume
of MC is 2.35 cm3 g−1. Besides, this material has a narrow pore
size distribution with an average pore diameter of 4−5 nm
(Figure 3b), as calculated by Barrett−Joyner−Halenda (BJH)
method. In contrast, the intermediate products have a BET
surface area of 40 m2 g−1 (as shown in Figure S1), which is
much smaller than that of the final product MC. This result
confirms that the KCl and NaCl salts formed during the
reduction serve as templates for the formation of the pores.
The intermediate product obtained after precursor reduction
was collected and characterized in order to confirm our
proposed reaction mechanism. The XRD pattern of the
Figure 3. (a) Nitrogen-sorption isotherm and (b) BJH pore-size
distribution of MC.
intermediate product (Figure S2) suggests the formation of
salts of NaCl and KCl. The TEM images of the intermediate
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ACS Applied Materials & Interfaces
The electrochemical performance of MC as a supercapacitor
electrode material was evaluated in two types of aqueous
electrolytes in coin cells, i.e., 6 M KOH (Figure 5) and 1 M
product (Figure 4a, b) show that the KCl nanocrystals with
diameter of 5−10 nm are observed within the carbon matrix,
Figure 5. (a) CV curves of MC at different scan rates ranging from 5
to 100 mV s−1 in 6 M KOH aqueous solution. (b) Galvanostatic
charge−discharge curves for MC at different current densities. (c)
Specific capacitance of MC as a function of the scan rate. (d)
Capacitance retention of MC measured at constant current density of
1 A g−1.
Figure 4. (a) Low-magnification TEM image of the intermediate
products, (b) HRTEM image of the intermediate products, (c) lowmagnification TEM image of MC, and (d) HRTEM image of MC.
Na2SO4 (Figure S5), with the optimal electrochemical
performance achieved in the electrolyte of 6 M KOH. Cyclic
voltammetry (CV) measurements were conducted at scan rates
from 5 to 100 mV s−1 to test the rate stability of the electrodes
(Figure 5a). Almost perfectly rectangular CV curves are
observed at all scan rates, indicating low resistance and high
power characteristics of the device. The galvanostatic charge/
discharge curves for MC at multiple current densities are shown
in Figure 5b. The specific capacitance of MC is 259, 236, 228,
220, 201, and 189 F g−1 at current densities of 1, 5, 10, 20, 50,
and 100 A g−1, respectively, which are calculated based on the
slope of the discharge curves (as shown in Figure 5c). To the
best of our knowledge, the capacitances of MC are among the
highest values reported for the porous carbon materials (as
shown in Table S1). The MC electrodes were also tested for
6000 charge/discharge cycles at 1 A g−1. As shown in Figure 5d,
92% of the original capacitance is retained after 6000 cycles,
indicating good cycle stability.
To gain insight into the capacitive performance of MC,
electrochemical impedance spectroscopy (EIS) was performed
in the frequency range of 10 mHz to 10 kHz at the open circuit
voltage with a voltage amplitude of 5 mV. The squashed
semicircle at high frequencies can be observed in the inset of
Figure 6. It indicates the faradaic characteristic of charge
transfer resistance at the electrode/electrolyte interface. The
equivalent series resistance of the MC electrode is only 0.53 Ω
(in term of the X-intercept of the Nyquist plots, as shown in
Figure 6 inset), which indicates the high electrical conductivity.
Besides, the short Warburg region indicates the short ion
diffusion paths in the electrode due to the existence of the
mesopore as an ion-buffering reservoir.26 The almost vertical
straight line at low frequencies features a nearly ideal double
layer capacitive behavior.27,28 The results above prove that MC
is a suitable material to be used as the electrode of the
electrochemical capacitor.
and the lattice fringes of KCl [200] with a d-spacing of 0.32 nm
are also clearly shown in the HRTEM image (Figure 4b). The
crystallite size of the salt is similar as the pore size of MC. The
SEM (Figure S4) and TEM images of MC (Figure 4c, d) show
typical morphology of MC with a disordered mesoporous
structure. Moreover, in Figure 4d, there are no salts left in this
mesoporous material. According to the XRD patterns, the BET
results and the TEM images, the self-forming templates could
be removed totally with just water.
A mechanism of the synthesis procedure is proposed based
on the results described above. In the first step, both salts (KCl
and NaCl) and carbon are produced and precipitated by the
reduction of CCl4 because of the low solubility of the salts in
toluene, as well as the fast production of large quantity of the
intermediate products (salts and carbon). Second, the
precipitation process and the aggregation process can lead to
formation of the intermediate product where the as-generated
salts aggregate and assemble with the reduction-generated
carbon into a composite. Finally, in the heat treatment, the salt
byproducts act as the template for the formation of mesoporous
structure in the carbon. After carbonization, the salts in the
intermediate products could be completely removed by water,
which makes this synthesis much easier than other hard
templating approaches. However, when the annealing temperature is increased to 800 °C, the salts would melt. As shown in
Figure S3, when the intermediate was thermally annealed at
800 °C, the BET specific surface area of mesoporous carbon
reaches 1205 m2 g−1, which is much lower than that of MC.
Moreover, the pore size calculated by the BJH method ranges
from 2.0 to 50 nm with a wide distribution. The low surface
area and the wide pore size distribution are both due to higher
annealing temperature.
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Figure 6. Nyquist plot of a typical MC electrode. The inset is an
enlargement of the high-frequency region of the Nyquist plot.
In summary, we have demonstrated a facile synthesis
procedure of the mesoporous carbon without any external
templates. Upon the reduction of the CCl4 precursor by NaK
alloy, carbon and salt byproducts are simultaneously produced,
precipitated and aggregated. The produced salts serve as
templates for the formation of the mesoporous carbon, and can
be easily removed by water, rather than harsh etchants, as is the
case for other templates. The produced mesoporous carbon has
a high BET surface area of 2012 m2 g−1 and a narrow pore-size
distribution, with an average pore diameter of 4 nm. As a
supercapacitor electrode, this mesoporous carbon exhibits high
specific capacitance (259 F g−1 at 1 A g−1) and high rate
stability (189 F g−1 at 100 A g−1). As shown in Figure 5d, 92%
of the original capacitance was retained after 6000 cycles,
indicating good cycle stability. This process opens a new
approach for the preparation of the mesoporous materials.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.5b12164.
Detailed experimental procedures, as well as XRD, SEM,
TEM, nitrogen adsorption/desorption isotherms, and
other supplemental data (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the Assistant Secretary for Energy
Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract
Numbers of DE-EE0006447 and DE-EE0005475.
■
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